THE CHALLENGE OF MILLENNIAL-SCALE QUATERNARY STRATIGRAPHY: THE POP (POLE-OCEAN-POLE) PROJECT

Millennial-scale climatic variability is best known from the records of the Greenland ice sheet, where rapid (within decades) temperature fluctuations of several degrees amplitude characterize the last glacial period. The reality of these events is certain because their existence has been independently verified in several different ice cores in an environment where continuity is reliable down to the annual scale. It is important that we learn the global mechanisms involved in these events, both because they are inherently remarkable, and because this may assist our prediction of man-made climatic change which will also be (geologically speaking) rapid. Even for the most recent of these events, where the uncertainties in radiocarbon dating are small in relation to their duration, correctly aligning records from other areas is challenging. Ice cores covering several glacial cycles have been recovered from Antarctica and demonstrate that important climatic variability is certainly not confined to the orbital (Milankovitch) frequencies. Ice cores preserve several independent records (air temperature, atmospheric composition, dust) so that the potential exists for high-resolution correlation with other continuous and detailed records by several routes. There are few if any deep-sea locations that preserve a real stratigraphy that is as continuous and detailed as the best Antarctic ice cores. As Parrenin will show in this session, there is the scope for developing high-resolution model time scales whose quality cannot be attained in any other geological deposits. Thus an important objective is to develop and test all possible tools for stratigraphic correlation to the long ice cores. A second is to iteratively optimize the Antarctic ice and air time scales on the basis of the increasing number of available cores. Finally, we must refine both the ice core time scale and the available radiometric dating methods so that there are no systematic offsets between the age estimates that they yield. In 2001 we collected several cores as part of a campaign to test the limits of correlation from the North Atlantic Ocean to Greenland and the Antarctic. A close sampling interval makes it possible to check the reliability of each brief event, and the existence of several cores permits the replicability of the events to be evaluated.